A conventional fuel cap 200 is shown in FIG. 6. Cap 200 comprises a cap body 201 formed from resin which is threadedly connected to the filler hole of a fuel tank, and a pressure control valve 203 mounted on the cap body 201 so as to control the pressure within the fuel tank. More specifically, the pressure control valve 203 comprises a rubber valve member 209 adapted to be seated on a seat surface 207 around a flow path hole 205 formed in the cap body 201, and a coil spring 211 for urging the valve member 209 in a valve closing direction. In this pressure control valve 203, the valve member 209 is opened and closed in accordance with the force acting on the valve member 209 (that is, the spring force) and a pressure differential (the fuel tank negative pressure) between atmospheric pressure and the pressure within the fuel tank.
An annular lip 213 is formed on the valve member 209 of the pressure control valve 203. The lip 213 is pressed against the seat surface 207 in a line contact manner, and also is elastically deformed to be held in intimate contact therewith, thereby enhancing the resulting seal.
FIG. 7 indicates the relation between the fuel tank negative pressure P and the flow rate Q of the pressure control valve 203 for the above fuel cap 200. From the viewpoint of pressure control, it is preferred that the valve be closed to keep the flow rate Q at zero when the fuel tank negative pressure P is below a predetermined pressure Pr, whereas when the fuel tank negative pressure P exceeds the predetermined pressure Pr, the valve should be opened to cause the flow rate Q to rise vertically as indicated with a solid line. However, typically, the conventional pressure control valve 203 has characteristics such that the flow rate Q gently rises in the vicinity of the predetermined pressure Pr as indicated with a broken line. Therefore, there has been a demand for the rise characteristics to be closer to the ideal characteristics as indicated with the solid line.
In the conventional pressure control valve 203, when a valve-closing pressure higher than the predetermined pressure Pr, that is, a positive pressure, is applied, the lip 213 is seated on the seat surface 207 by the urging force of the coil spring 211, thereby ensuring the sealing effect. With this sealing construction, however, unless the dimensional accuracy of the lip 213 of the valve member 209, as well as the surface accuracy of the seat surface 207, is great, the characteristics are gentle as indicated with the broken line, which results in the problem that satisfactory sealing can not be obtained.
That problem is encountered not only with the above negative pressure type pressure control valve 203 which is opened when the fuel tank negative pressure P becomes great, but also with a pressure control valve of the positive pressure type which is opened when the pressure within a fuel tank exceeds atmospheric pressure by a predetermined amount.